This invention relates to a marine structure for installation in waters upon which thick sheets of ice are formed during the winter season. In Arctic and Antarctic waters the winter ice normally may reach thicknesses of 6 to 10 feet or more and rafting, pressure ridges, and other accumulations may cause the thickness of the ice in places to be several times the thickness of the original sheet. The ice sheets are of vast areas; and, although normally they may move relatively slowly with wind and water currents, the mass of ice in movement can cause very high forces on a stationary structure in its path. Such ice may have a compressive strength in the range of about 650 to 1000 pounds per square inch and a structure strong enough to withstand the crushing force of the ice would necessarily be very massive and correspondingly expensive to construct.
It has been proposed heretofore that rather than build a structure strong enough to withstand the total crushing force of the ice, i.e., strong enough to permit the ice to be crushed against the structure and thus enable the sheet of ice to flow around it, the structure be built with ramp-like surfaces which would cause an edge of the moving ice sheet to be forced upwardly above its normal position on the surface of the water as it came into contact with the structure, thus bending the ice sheet and placing a tensile stress in the ice. Since the ice has a flexural strength of about 85 pounds per square inch, a correspondingly relatively smaller force is placed on the structure as the ice impinging on it fails in tension.
Several forms of structures having a sloping perimetrical wall for installation in waters where they would be exposed to the forces of moving ice are illustrated in a paper by J. V. Danys entitled "Effect of Cone-Shaped Structures on Impact Forces of Ice Floes," presented to the First International Conference on Port and Ocean Engineering under Arctic Conditions, held at the Technical University of Norway, Trondheim, Norway, during Aug. 13-30, 1971.
Another publication of interest in this respect is a paper by Ben C. Gerwick, Jr., and Ronald R. Lloyd entitled "Design and Construction Procedures for Proposed Arctic Offshore Structures," presented at the Offshore Technology Conference meeting at Houston, Texas, during Apr. 1970.
In testing in a laboratory cold room, scale models of offshore structures incorporating the above design principle to investigate the action upon them of sheet ice, it was found that the ramp-type of surface, when moving relative to the ice sheet and in contact with it, caused appreciably less force to be imposed on the platform structure than would be the case if the platform wall presented to the ice sheet was disposed vertically to it as would be the situation if, for example, a proportionately larger-diameter pile or caisson was contacted by the moving ice sheet. It was discovered, however, that this condition was true only while the ice sheet could move relative to the platform and that, as explained hereinafter, ordinarily much larger forces would be imposed on the marine structure before the bond between it and the ice was broken to permit such relative movement.
In the actual installation of a marine structure in Arctic waters, it is proposed to construct and assemble the structure in a shipyard and tow the assembled structure to the offshore site, where it will be established during the time the waters are open and relatively ice-free. At this time, the structure will be lowered into contact with the submerged earth and piles may be driven into the earth to hold the structure in place against the horizontal forces imposed upon it. Piles may also be used to assist supporting the vertical loads on the structure.
In the farther-north Arctic waters, such as the waters off the North Slope of Alaska, the open-water season is relatively short, approximately 6 weeks, after which ice begins to form on the open waters. The ice will freeze around and onto the marine structure which has been fixed at the offshore site. This condition has been duplicated in the laboratory to determine what effect the newly frozen ice sheet would have on a scale model of a ramp-sided offshore structure, as described heretofore, and particularly to determine what forces would be imposed on it.
As the ice sheet built up in thickness on the surface of the water surrounding the model structure, it also froze onto the surface of the structure in contact with the water. When the ice sheet reached the required thickness for the test, it was found that a much greater force was required to start relative motion between the model and the adhering ice sheet than was required to maintain the relative motion after the adhesional bond was broken. For the conditions of the test, approximately 5 to 10 times as much force, depending on specific conditions, was imposed on the model structure by the ice sheet before the bond was broken than was imposed after this relative motion was established.
The amount of force imposed initially on the structure by the ice sheet will, of course, be dependent on the form, dimensions and characteristics of the structure and the dimensions and characteristics of the ice. But, in all cases, as the problem is understood now, a much greater force will be imposed initially on the structure before the adhesional bond between it and the ice is broken than will be imposed after the bond is disrupted. Ordinarily it would be necessary under these conditions to build the structure strong enough to withstand the initial forces imposed on it by the ice sheet, even though the forces imposed on it during the major portion of its useful life would not require a structure of such rugged construction. A structure built strong enough to withstand the initial ice forces would be correspondingly more expensive to build and more difficult to install than one designed to take only the load of a relatively moving ice sheet. The present invention is designed to alleviate this condition of initial high loading imposed on the offshore structure by a method and apparatus to be described hereinafter.